1. The field of The Invention
This invention concerns a system and process for controlling the operation of the calciner system.
2. State of The Art
A calciner is a device for heating material in order to bring about physical or chemical reactions in the material to produce a product. One conventional type of calciner is a rotary, co-current calciner. Such a calciner includes a rotatable cylinder for containing the material during the heating and reaction process. The rotatable cylinder is slightly inclined and coupled to the upper end of the cylinder is a stationery heating plenum which contains a burner to burn fuel to thereby provide heat for the calcining process. A forced draft fan is coupled to the heating plenum to provide air for combustion in the burner and also to carry heated gases through the rotating cylinder. At the opposite, lower end of the rotatable cylinder, a stationary receiving duct is coupled. The receiving duct has a screw conveyer disposed at its lower end so that calcined material leaving the rotating cylinder falls into the screw conveyer to be conveyed to storage. An induced draft fan is coupled to the upper end of the receiving duct to pull flue gases from the rotating cylinder and the receiving duct.
One conventional use for a co-current calciner such as described above is to calcine sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) to soda ash (Na.sub.2 CO.sub.3). When sodium sesquicarbonate is converted to soda ash in a conventional calcining process, the following reactions occur. EQU Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O+Heat.fwdarw.Na.sub.2 CO.sub.3 +NaHCO.sub.3 +2H.sub.2 O (vapor) EQU 2Na.sub.2 CO.sub.3 +2NaHCO.sub.3 +4H.sub.2 O (vapor)+Heat.fwdarw.3Na.sub.2 CO.sub.3 +5H.sub.2 O+CO.sub.2 +Heat EQU 3Na.sub.2 CO.sub.3 +5H.sub.2 O+CO.sub.2 +Heat.fwdarw.(3-x)Na.sub.2 CO.sub.3 +xNa.sub.2 O+5H.sub.2 O+(1+x)CO.sub.2
In the conversion of sodium sesquicarbonate to soda ash, the extent to which the sodium sesquicarbonate is heated is important. If insufficient heat is applied, all of the sodium bicarbonate (NaCO.sub.3) is not converted to soda ash. On the other hand, if too much heat is applied, soda ash is converted to sodium oxide (Na.sub.2 O) which is not a desirable end product. Thus in conventional processes for calcining sodium sesquicarbonate, the product resulting from the calcining operation is periodically taken to a laboratory and analyzed to determine the amount of sodium bicarbonate and sodium oxide in the products. In the laboratory analysis the sample is dissolved in water and analyzed on a wet basis. The following reactions occur: EQU Na.sub.2 O+H.sub.2 O.fwdarw.2NaOH EQU NaOH+NaHCO.sub.3 .fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O
Thus Na.sub.2 O and NaHCO.sub.3 react with one another to produce Na.sub.2 CO.sub.3 and any excess Na.sub.2 O is reported as NaOH, while NaHCO.sub.3 is measured directly. Likewise, in the calcining process itself the soda ash is often used in a water solution so that the same reactions occur. During laboratory analysis, if either sodium hydroxide or sodium bicarbonate is found in excessive quantities, then the calcining process is altered. Often, control of the process is accomplished by controlling the fuel supplied to the burner and controlling the amount of air supplied by the forced draft fan.
It can be seen that the laboratory measurement of the sodium bicarbonate and sodium oxide in the product is not a completely satisfactory basis for controlling the process. This is because the measurement takes substantial time and therefore large quantities of product can be produced which contain too much sodium bicarbonate or sodium oxide. Furthermore, the application of excessive heat to the material in the calciner is undesirable because valuable fuel is wasted, but if laboratory analysis is used to determine the sodium oxide in the product, large amounts of heat can be wasted before the process is controlled.